Equipment for producing a three-dimensional object layer by layer using a powdery material which can be solidified, or fused together, by irradiating it with a high-energy beam of electromagnetic radiation or electrons are known from e.g. U.S. Pat. Nos. 4,863,538 and 5,647,931 and Swedish Patent No. 524,467. Such equipment includes for instance a supply of powder, means for applying a layer of powder on a vertically adjustable platform or working area, and means for directing the beam over the working area. The powder sinters or melts and solidifies as the beam moves over the working area.
When melting or sintering a powder using a high-energy beam, it is important to avoid exceeding the vaporization temperature of the powder, since otherwise the powder will merely vaporize instead of forming the intended product. U.S. Patent Publication No. 2005/0186538 discloses a method focusing on this problem. In this method a laser beam is repeatedly directed to the same powder target area during the melting/sintering phase so as to stepwise raise the powder temperature. In this manner, too high a powder temperature is avoided.
When using an electron beam instead of a laser beam, the situation is in some ways different. As the electron beam hits the powder, a charge distribution develops around the electron target area. If the charge distribution density exceeds a critical limit, an electrical discharge will occur since the powder particles will repel each other. The result of such a discharge is that the structure of the powder layer will be destroyed. Applying the method according to U.S. Patent Publication No. 2005/0186538 to a powder melting/sintering device equipped with an electron beam is likely to give a poor result since no measures are taken in that method to avoid such discharges.
One solution to the problem of avoiding discharges is to add conductive material, such as carbon, to the powder so as to increase the electrical conductivity of the powder. Disadvantages of this solution are, however, that the solidifying process of such a powder mixture may be difficult to control and that the properties of the formed product may be affected in a negative way. For instance, the mechanical strength may be decreased.
One object of the present invention is to provide a method and apparatus for the layered production of three-dimensional objects from a powdery material, which method and apparatus allow for a controlled and proper fusing together of the powdery material, and which are well suited for both an electron beam and a laser beam.